Cushing — Geology of Clinton County. 



511 



is unquestionably near the summit of the formation. In Dannemora it is 

 found at an elevation in the neighborhood of 1,500 feet, and in Ellenburgh, 

 Beekmantown and Altona it runs nearly as high. Here, moreover, it is the 

 base which is exposed. 



The color is very varied, from white through various greys, yellows, 

 browns and reds to even blackish appearing layers. Occasionally greenish 

 layers, appearing somewhat glauconitie, occur. In general the reds prevail in 

 the lower beds and are scarce above. The blackish varieties appear to be 

 characteristic of the summit. 



The main mass of the formation is made up of quite pure quartz sand. 

 In the basal layers there is a large feldspathic content, mostly brthoclase, but 

 with a little plagioclase and microcline. Occasional grains of pyroxene and 

 hornblende and scales of biotite appear. In some layers zircon is prominent. 

 Garnet has not yet been noted. The basal conglomerates are often very rich 

 in magnetite. Toward the summit, dolomitic layers appear, regularly alter- 

 nating with those of pure sand, and forming passage beds to the Calciferous 

 above. Sometimes these are of pure dolomite, but prevailingly they are 

 sandy, and the rather large rounded grains of quartz embedded in a fine 

 mosaic of minute dolomite crystals furnish a striking combination in thin 

 section. These dolomitic beds are not, however, confined to the higher layers, 

 but occur sparingly elsewhere, even among red beds well toward the base, 

 strata of such an appearance that it would scarcely occur to one to test them 

 with acid ; in fact, the presence of these layers was first made known by 

 thin sections. 



When the base of the formation is neared, massive conglomerates become 

 prominent, sometimes attaining great coarseness. They are commonly inter- 

 stratified with finer red, feldspathic. easily weathering beds, which are much 

 permeated with hematite. It is a curious fact that the materials of which 

 these basal conglomerates are made up are, so far as observed, always of 

 gneissic origin, even when the contacts are with gabbro and a considerable 

 distance from known gneissic outcrops.* 



Along the lake these basal beds are seldom exposed, the contacts shown 

 being largely brought about by faults, but on the north they have great 

 prominence, especially in Ellenburgh (as noted by Emmons f), where they 

 have considerable thickness, but by no means make up the whole mass, as 

 stated by -him.- 1 



;m yHi.l «• ■ ;•■• 



* See description of a conglomerate near Keeseville on a following page, 

 t E. Emuions. -Geology of New York, Second District, p. 309. 



